1. Field of the Invention
The present invention relates to a wavelength-dependent compensation method in a variable optical attenuator, and in particular to a wavelength-dependent compensation method in a variable optical attenuator which attenuates the wavelength of an optical signal over a wide range of wavelengths in the optical signal, measures the power of the optical signal with attenuated wavelengths, and supplies compensation data which compensates wavelength-dependent loss.
2. Description of Related Art
It is known that the amount of attenuation of a natural density filter (herebelow, abbreviated "ND filter") generally used to suppress the amount of light in a variable optical attenuator depends on wavelength characteristics. In order to achieve an accurate setting of the amount of attenuation, it is necessary to limit attenuation to a single wavelength, to actuate an ND filter with a motor drive member, to produce rotation of the motor by the input of the pulse of its motor drive, and either to compensate only the number of pulses corresponding to the angle of rotation of the motor with respect to the amount of attenuation, or to compensate the entire range of wavelengths to be set by using compensating data for the amount of attenuation.
The method of compensating wavelength-dependency of an ND filter used in conventional variable optical attenuators is explained in FIG. 9.
FIG. 9 is a block diagram showing a structure of a variable optical attenuator applying a conventional compensation method for wavelength-dependency. In FIG. 9, an optical signal 6a that transits ND filter 1 is attenuated so as to equal the preset value of the amount of attenuation set by ND filter 1 and is transformed into attenuated optical signal 6b. The motor 2 is actuated by motor drive member 3 under control of a CPU 4.
If the wavelength of optical signal 6a transiting ND filter 1 is uniform, the amount of attenuation of the ND filter 1 is proportional to the angle of rotation, and the angle of rotation of motor 2 is proportional to the pulse number input into the motor drive member 3.
However, as is clear from the wavelength-characteristics graph of an ND filter 1 shown in FIG. 2, where the abscissa is the pulse number and the ordinate is the amount of attenuation (dB), generally in the attenuation of an ND filter 1, the correction of the amount of attenuation is conventionally limited to a single wavelength (e.g., 1310 nm) because there is wavelength dependency.
Therefore, an accurate amount of attenuation of an optical signal differing from the wavelength used for correction (e.g., 1310 nm) could not be attained.
Because of this, when precisely setting the amount of attenuation with a conventional variable optical attenuator, it is necessary to measure for each wavelength, the relationship between the amount of attenuation of an ND filter 1 and the number of pulses corresponding to the angle of rotation of motor 2, and store the compensating data for each wavelength in the compensating ROM 5 (see FIG. 9) under the control of a CPU 4. However, as can be understood from the linear graph of an ND filter 1 in FIG. 2, used when we explain below the embodiments of the present invention, at each wavelength the relationship between the amount of attenuation of an ND filter 1 and the number of pulses corresponding to the angle of rotation of motor 2 are wavelength dependent (i.e., the linear slope for each wavelength is different).
In addition, from the wavelength characteristic graph of an ND filter in FIG. 3, where the abscissa is the wavelength (nm) and the ordinate is the amount of attenuation (dB), the problem arises that even when the preset value of the amount of attenuation is near 0 dB (insertion loss), there is wavelength-dependency.
Therefore, in the conventional variable optical attenuators, in accurately presetting the amount of attenuation with respect to an ND filter 1, the wavelength is limited to a single wavelength, and either only the pulse number corresponding to the angle of rotation (see FIG. 9) of motor 2 is compensated, or the entire range of wavelengths to be preset are compensated using compensating data of the pulse number corresponding to the angle of rotation of motor 2 with respect to the amount of attenuation of an ND filter 1.
However, in this case, for an optical signal whose wavelength differs from the wavelength (in FIG. 2, 1310 nm) used in correction, accurate amounts of attenuation cannot be obtained, and the problems arise that the number of observation operations for compensating data becomes huge, the amount of memory allocation in the compensation ROM 5 for the variable optical attenuator becomes very large, and the cost becomes very high.